For part a:
The time between each dot is constant (1/30 seconds) and the horizontal distance between each dot is also constant (horizontal velocity stays the same in projectile motion), so you just have to measure the horizontal distance between any two dots (and adjust for the scale) and then multiply it by 30 to get the distance it would travel in a second (aka the velocity)
For part b:
Now that you know the horizontal velocity, you can measure the horizontal distance between the first dot shown and the start, and then divide it by the velocity to get the time. Once you’ve done this, you can multiply the time by 30 (the number of dots created per second) to get the dots missing from that time gap.
Part c:
This is the same as b, because they both start with 0 vertical velocity, experience the same gravity and have their first dot at the same height.
